The present invention is directed to a fiber optic network having reduced coupling losses, and more particularly to a fiber optic network wherein wavelength multiplexing and demultiplexing are employed to reduce optical losses.
Continuing improvements in the transmission quality of optical fibers, and in particular increased bandwidth and reduced attenuation rates, have made optical fiber communication networks an increasingly attractive alternative to networks which employ conductors as the transmission medium. In order to communicate optically, an electrical signal developed within a transmitting terminal device, such as, for example, a telephone, computer, or numerically controlled machine tool, is delivered to an optical transmitter within the terminal device. The optical transmitter uses the electrical signal to modulate light from a source such as an LED or laser. The modulated light is then transmitted via an optical fiber to an optical receiver within a receiving terminal device. The optical receiver includes an optical detector, such as a photodiode, which reconverts the modulated optical signal to an electrical signal. Thus the optical transmitters and receivers within the terminal devices and the optical fibers connecting them effectively replace conductors which might otherwise have been used. Optical communication networks are particularly useful for transmitting digital data in serial form.
Various devices are known for splicing optical fibers or otherwise manipulating optical signals. For example, a transmissive fiber optic star is a passive coupling device used to interconnect a number of terminal devices in a network. In such a network provisions must be made to prevent signals emitted by the optical transmitters of different terminal devices from interfering with each other. Such interference can be avoided by several sophisticated digital communication techniques which have been developed in the electrical communication art, and which can be implemented by circuitry within the terminal devices. In the "token passing" network control system, for example, a terminal having access to the network transmits any message it may have and then "passes the token" by emitting a code identifying the next terminal device entitled to access for the purpose of transmitting a message. The token is passed through a sequence which includes each terminal device in the network, thereby affording equitable access to every terminal device. In the "collision detection" network control system, on the other hand, each terminal device monitors the network and is allowed to transmit a message when the network is not busy. This occassionally results in a "collision" caused by two or more terminal devices that transmit almost simultaneously. In such a situation, each terminal device aborts its message and tries again after a random delay.
Splitters and couplers, like stars, are devices for optically joining fibers. FIG. 1A illustrates a passive splitter 10, which receives light via incoming fiber 12 and disperses the light to outgoing fibers 14 and 16. Passive coupler 18 in FIG. 1B may simply be a splitter used in reverse. Coupler 18 receives light on incoming fibers 20 and 22 and produces a combined output on fiber 24.
The laws of optical propagation are such that passive splitters and couplers inherently introduce significant optical losses. For example if one unit of optical power enters fiber 12 of splitter 10, the power available at each of output fibers 14 and 16 is only half a unit. Thus, there is a 3 dB loss between input fiber 12 and either output fiber 14 or 16. The same phenomenon occurs in coupler 18. If the optical power applied to each of incoming fibers 20 and 22 is one unit, so that the total optical power delivered to coupler 18 is two units, the output on fiber 24 is only one unit. Here again only half of the power available on each of incoming fibers 20 and 22 appears at the output, so that there is a minimum of a 3 dB loss that is inherent in the device. The power losses which are inherently present in passive splitters and couplers can be avoided by configuring a splitter as a wavelength demultiplexer and by configuring a coupler as a wavelength multiplexer. An optical or wavelength demultiplexer is a device which receives an incoming beam of light at various wavelengths and spatially separates the component wavelengths, so that there is an outgoing beam at each wavelength. An optical multiplexer performs the reverse operation. In an optical multiplexer, spatially separated beams of incoming light at different wavelengths are merged to produce a combined outgoing beam. Optical multiplexers and demultiplexers are commercially available devices, some using diffraction gratings, some using mirrors and filters to combine and separate light by wavelength. The role of mirrors and filters in manipulating light by wavelength is summarized in "International Fiber Optics and Communications Handbook and Buyers Guide," Volume 5, 1983 edition, pages 34-41, published by Information Gatekeepers, Inc. of 167 Corey Road, Brookline, Mass. 02146.
If splitter 10 were based upon wavelength demultiplexing and incoming fiber 12 carried optical signals at two different wavelengths, the signal at one wavelength would be routed to outgoing fiber 14 and the signal at the other wavelength would be routed to outgoing fiber 16. The point of importance here is that the power level of the signal on outgoing fiber 14, for example, would be the same (neglecting minor losses) as the power at that wavelength in fiber 12. Similarily, virtually all of the optical power at the other wavelength would be conveyed from fiber 12 to fiber 16. Thus a wavelength splitter based upon optical demultiplexing would split an incoming signal into different wavelengths without attenuating the power levels of the components. The same would be true of a coupler based upon wavelength multiplexing. If coupler 18 were a wavelength multiplexer and if fiber 20 carried light at a first wavelength and fiber 22 carried light at a second wavelength, the combined output on fiber 24 would consist of both wavelengths, with the total optical power being the same as the power received, at different wavelengths, via fibers 20 and 22.
In the remainder of this application the terms "splitter" and "coupler" will refer to devices which separate or combine light by wavelength, and avoid the inherent loss associated with broadband passive splitters and couplers.